Oxidation behavior of fine-grain MA 956 superalloy

This investigation was undertaken in an attempt to gain a fundamental under standing of the oxidation behavior of a fine-grain MA 956 in the temperatur e range 800 to 1200 degrees C, with emphasis placed on the scale morphology and oxidation kinetics. Oxidation reaction led to the formation of a thin and dense alumina scale, irrespective of the oxidation temperature. At inte rmediate temperatures plateletlike oxides of alumina, probably theta-Al2O3, covered the surface, whereas at other temperatures small nodules of Fe-, C r-, Ti or Y-rich oxides were observed in the outer part of the alumina scal e. The temperature dependence of the parabolic rate constant revealed a cha nge in the oxidation mechanism with a transition at about 1000 degrees C. T he high-temperature mechanism is controlled by the formation of alpha-alumi na, whereas the low-temperature oxidation mechanism is controlled by format ion of metastable alumina. Comparison with the reported oxidation kinetics for coarse-grain MA 956 is inconclusive, since microstructure (grain size, texture) of the substrate changed during oxidation in the high-temperature range.